Production of nitrosamine-free silicone articles

ABSTRACT

The present invention relates to a method for making nitrosamine-free silicone articles by treating vinyl-containing organopolysiloxanes with an effective amount of an acid.

The present invention relates to production of nitrosamine-free siliconearticles. More particularly, it is concerned with production ofnitrosamine-free silicone articles by means of filler treatment.

BACKGROUND OF THE INVENTION

Several reports have been published describing the presence of volatileN-nitrosamines in various rubber products. The present concern about theoccurrence of volatile N-nitrosamines in baby bottle rubber nipples andthe possible migration of these compounds into infant formula wasprompted by a report of Preussmann et al., (1981) Am. Chem. Soc. Symp.Ser. 174, American Chemical Society, Washington, D.C., p. 217.

A method was described for the estimation of volatile N-nitrosamines inthe rubber nipples of babies bottles. In study of rubber nipples fromone manufacturer, N-nitrosodimethylamine, N-nitrosodiethylamine andN-nitrosopiperidine were determined by gas chromatography, using athermal energy analyser, and their presence was confirmed by massspectrometry with average levels of individual nitrosamines ranging from22 to 281 ppb. When the nipples were sterilized in a conventionalsterilizer together with milk or infant formula the three nitrosaminesmigrated into the milk or formula. Storing a bottle of milk with arubber nipple inverted in it for 2 hr at room temperature or overnightin a refrigerator after sterilization resulted in an 8-13 % averageincrease in the nitrosamine levels migrating into the milk. On repeatedsterilization of a single nipple, the quantities of nitrosaminesmigrating into milk from rubber nipples declined steadily, but afterseven sterilizations, nitrosamines were still readily detectable in themilk. Nitrosamine levels were higher in rubber nipples aftersterilization, indicating the presence of nitrosamine precursors in thenipples. No nitrosamines were found in raw, uncured rubber. Chemicalaccelerators and stabilizers added during the vulcanization process arethe source of the amine precursors in rubber nipples.

On Jan. 1, 1984, the U.S. Food and Drug Administration (hereinafter"FDA") established an action level of 60 ppb total N-nitrosamines inrubber nipples. The action level was reduced to 10 ppb on Jan. 1, 1985.

A collaborative study was conducted on the FDA dichloromethaneextraction method for determining volatile N-nitrosamines in baby bottlerubber nipples. Following dichloromethane extraction, N-nitrosamineswere determined by gas chromatography-thermal energy analysis. Six pairsof blind duplicate rubber nipple samples representing 6 lots wereanalyzed by 11 collaborating laboratories. All samples were portionstaken from equilibrated composites of cut-up rubber nipples obtainedfrom manufacturers in the United States. Recoveries of the internalstandard (N-nitrosodipropylamine) at approximately 20 ppb ranged from 10to 120%. Reproducibility relative standard deviations (RSD) were between35 and 45% for N-nitrosamine levels from 10 to 20 ppb. However, whendata from laboratories with recoveries less than 75% were excluded (thisis now specified in the method), RSD, values were between 11 and 32% forN-nitrosamine levels from 6 to 26 ppb. Values were consistent with orbetter than those reported for other analytical techniques designed toquantitate trace contaminants at the low ppb level, e.g., aflatoxin infood. The method has been adopted official first action for thequantitation of volatile N-nitrosamines in baby bottle rubber nipples.See Gas Chromatographic-Thermal Energy Analysis Method for Determinationof Volatile N-Nitrosamines in Baby Bottle Rubber Nipples: CollaborativeStudy, by Gray & Stachiw, J. Assoc. Off. Anal. Chem. (1987) 70, MarchIssue.

Although research in the rubber industry has been devoted to lowering oreliminating nitrosamines, none of these studies have included siliconerubber materials. Silicone elastomeric compositions, in contrast tosynthetic rubber compounds, are usually prepared from a vinyl-containingpolydiorganosiloxane, an organohydrogensilicone crosslinker, and aplatinum catalyst. The compositions of this type are desirable for manyreasons. For instance, they cure without by-products. They can cure atroom temperature or at elevated temperatures. They can be stabilized forstorage at room temperature by utilization of a platinum catalystinhibitor. And, they can be made from high and low viscosity polymers.These compositions utilize components that are low enough in viscositythat they are easily pumpable or extrudable as well as have a fast curetime. These compositions also provide cured silicone elastomers whichare characterized by high strength and high durometer.

Cross-linked silicone polymers with their particularly lowintermolecular interactions have low tensile strengths. Only with theaddition of reinforcing fillers can high-strength silicone polymers beobtained. Particularly suitable are fumed silicas with BET surface areasof 150 to 400 m² /g which increase the tensile strength about 20 fold to10-12 MPa. At the same time, viscosity is considerably increased becausefumed silicas have a strong thickening effect. This effect is caused byformation of agglomerates of the primary silica particles. Theseagglomerates build a three-dimensional network (tertiary structure) viahydrogen bonds so that the bulk density of the fumed silica is onlyabout 50 g/l. To produce a mixture of 40 parts silica, in 100 partspolymer therefore requires addition of 8 volume parts of filler for 1volume part of polymer. These ratios clearly indicate the necessity ofusing treating agents which reduce interactions between filleragglomerates as much as possible. The most effective and most commonlyused treating agents is hexamethyldisilazane (hereinafter "HMDZ"). Thefillers treated with HMDZ have a considerably reduced thickening effectand therefore are particularly suitable for the use in liquid siliconerubbers.

Since silicone elastomer are entirely different polymers, thesesilicones became leading candidates to replace the synthetic rubbercompounds, Analysis of the cured silicone elastomers showed no presenceof nitrosamines. However, to applicants' surprise, upon post-baking asrequired by FDA, the presence of nitrosamines was detected. Forsilicones to serve these FDA regulated markets, a method of removing thenitrosamines must be found.

SUMMARY OF THE INVENTION

There is provided by the present invention a method for makingnitrosamine-free silicone articles comprised by mixing (A)vinyl-containing organopolysiloxane; (B) silicon hydride siloxane; (C)filler; (D) a catalytic amount of a platinum metal group compound or aperoxide and (E) an effective amount of acid, curing and post baking themixture.

The critical feature that led to this invention is based on thediscovery that if the polymer/filler mixture is treated with aneffective amount of acid, the resultant part is substantially free ofnitrosamine.

DETAILED DESCRIPTION OF THE INVENTION

Component (A), the vinyl-containing organopolysiloxanes, generally has aviscosity of from 5,000 to 1,000,000 centipoise at 25° C. The preferredvinyl-containing organopolysiloxanes are vinyl-stopped polymer havingthe general formula M^(Vi) D_(x) M^(Vi), vinyl-on-chain copolymers suchas MD^(Vi) _(x) D_(y) M, vinyl-stopped, vinyl-on-chain copolymers suchas M^(Vi) D_(x) D^(VI) _(y) M^(Vi), vinyl and trimethylsilyl-stoppedcopolymers such as MD_(x) M^(Vi), or a mixture thereof, wherein Virepresents a vinyl radical, M represents a trimethylsiloxy radical,M^(Vi) represents dimethylvinylsiloxy, D is dimethylsiloxy. Suchpolymers are taught by U.S. Pat. Nos. 5,082,886, 4,340,709, 3,884,866issued to Jeram et al., U.S. Pat. No. 5,331,075 issued to Sumpter etal., U.S. Pat. No. 4,162,243 issued to Lee et al., U.S. Pat. No.4,382,057 issued to Tolentino, and U.S. Pat. No. 4,427,801 issued toSweet, hereby incorporated by reference.

Component (B), the silicon hydride siloxane or silicon hydride siloxanefluid used in the invention can have about 0.04 to about 1.4 % by weightof chemically combined hydrogen attached to silicon. One form of thesilicon hydride siloxane is a "coupler" having the formula, ##STR1##where R¹ is selected from C₁₋₁₃ monovalent hydrocarbon radicals free ofolefinic unsaturation and n is an integer having a value sufficient toprovide the "coupler" with a viscosity of 1 to 500 centipoises at 25° C.and from about 3 to 9 mole percent of chain-stopping diorganohydridesiloxy units, based on the total moles of chemically combined siloxyunits in the silicon hydride siloxane fluid.

In addition to the silicone hydride coupler of formula (1), the siliconhydride siloxane fluid used in the heat curable organopolysiloxanecompositions of the present invention also can include silicon hydrideresins consisting essentially of the following chemically combinedunits, ##STR2## chemically combined with SiO₂ units, where the R² +H toSi ratio can vary from 1.0 to 2.7. Silicon hydride resin also can haveunits of the formula, ##STR3## chemically combined with SiO₂ units and(R⁴)₂ SiO units, where the R³ +R⁴ +H to Si ratio can vary from 1.2 to2.7, where R², R³ and R⁴ are C₁₋₁₃ monovalent hydrocarbon radicals freeof olefinic unsaturation selected from R¹ radicals.

The silicon hydride siloxane fluid also can include linear hydrogencontaining polysiloxane having the formula, ##STR4## where R⁵ is a C₁₋₁₃monovalent hydrocarbon radical free of olefinic unsaturation, selectedfrom R¹ radicals, and p and q are integers having values sufficient toprovide a polymer having a viscosity of from 1 to 1,000 centipoises at25° C.

In formulas (1) and (2) and the chemically combined units describedabove, R¹, R², R³, R⁴ and R⁵ can be the same or different radicalsselected from the group consisting of alkyl radicals of 1 to 8 carbonatoms, such as methyl, ethyl, propyl, etc.; cycloalkyl radicals such ascyclohexyl, cycloheptyl, etc.; aryl radicals such as phenyl, tolyl,xylyl, etc.; and haloalkyl radicals such as 3,3,3-trifuloropropyl.

Component (C), the filler is any reinforcing or extending filler knownin the prior art. In order to get the high tensile strength, forexample, a reinforcing filler is incorporated. Illustrative of the manyreinforcing fillers which can be employed are titanium dioxide,lithopone, zinc oxide, zirconium silicate, silica aerogel, iron oxide,diatomaceous earth, calcium carbonate, fumed silica, silazane treatedsilica, precipitated silica, glass fibers, magnesium oxide, chromicoxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay,asbestos, carbon, graphite, cork, cotton, synthetic fibers, etc.

Preferably, the filler is either a fumed or precipitated silica that hasbeen treated. The treating process may be done in accordance with theteachings of U.S. Pat. No. 4,529,774 issued to Evans et al., U.S. Pat.No. 3,635,743 issued to Smith, U.S. Pat. No. 3,847,848 issued to Beers;hereby incorporated by reference, Alternatively, and most preferably,the filler is treated in-situ; that is the untreated silica filler andthe treating agents are added to the silicone elastomer compositionseparately, and the treatment process is accomplished simultaneouslywith the mixture of the filler into the elastomer. This in-situ processis taught by Evans in U.S. Pat. No. 4,529,774; hereby incorporated byreference.

Alternatively, the fillers can be replaced by the vinyl treated silicafiller of U.S. Pat. No. 4,162,243 issued to Lee et al.; and U.S. Pat.No. 4,427,801 issued to Sweet; hereby incorporated by reference.

Component (D), the catalyst, is any compound that promotes thehydrosilation reaction between a silicon hydride and an ethylenicallyunsaturated polyorganosiloxane. Typically, it is a precious metalcompound; usually platinum. Such catalysts are well known in the art.Preferred catalysts are taught by in U.S. Pat. Nos. 3,917,432, 3,197,433and 3,220,972 issued to Lamoreaux, U.S. Pat. Nos. 3,715,334 and3,814,730 issued to Karstedt, and U.S. Pat. No. 4,288,345 issued toAshby et al., hereby incorporated by reference.

Alternatively, the catalyst can be a peroxide or it can be a combinationof peroxides comprising a low temperature peroxide and a hightemperature peroxide.

Since mixtures containing Components A, B, and C with the catalyst,Component D, may begin to cure immediately on mixing at roomtemperature, it may be desirable to inhibit the action of the catalystat room temperature with a suitable inhibitor if the composition is tobe stored before molding. Platinum catalyst inhibitors are used toretard the catalytic activity of the platinum at room temperature, butallow the platinum to catalyze the reaction between Components A, B andC at elevated temperature.

One suitable type of platinum catalyst inhibitor is described in U.S.Pat. No. 3,445,420 issued to Kookootsedes et al. which is herebyincorporated by reference to show certain acetylenic inhibitors andtheir use. A preferred class of acetylenic inhibitors are the acetylenicalcohols, especially 2-methyl-3-butyn-2-ol.

A second type of platinum catalyst inhibitor is described in U.S. Pat.No. 3,989,667 issued to Lee et al. which is hereby incorporated byreference to show certain olefinic siloxanes, their preparation andtheir use as platinum catalyst inhibitors.

A third type of platinum catalyst inhibitor is apolymethylvinylcyclosiloxane having three to six methylvinylsiloxaneunits per molecule.

The optimum concentration of platinum catalyst inhibitor is that whichwill provide the desired storage stability at ambient temperaturewithout excessively prolonging the time interval required to cure thecompositions at elevated temperatures. This amount will vary widely andwill depend upon the particular inhibitor that is used, the nature andconcentration of platinum-containing catalyst and the nature of theorganohydrogensiloxane.

The mixture of component (A) and component (C) is treated with component(E) an effective amount of acid. The acid used in this invention can beany acid which is compatible with silicone, such as formic acid, aceticacid, phosphoric acid, HCl, HBr, HI sulfuric, etc. The acid is added topolymer/filler mixture and the mixture is cooked at temperatures ofabout 50° to about 100° C. for 0.5 to 2.0 hours, preferably attemperatures of about 55° to about 90° C. for 0.5 to 1.5 hours, and mostpreferably at temperatures of about 60° to about 85° C. for one hour.The resulting mixture is then stripped until the system is substantiallyvolatile free.

Compositions of the present invention can be used in a liquid injectionmolding process in which the composition is injected into light weightmolds under low pressures, such as 600 kPa cylinder pressure. Suchcompositions can be cured very rapidly in a hot mold and removed withoutcooling the mold. The type of molding, extruding or curing process usedis not narrowly critical and can include those known in the art. Anadvantage of the compositions of this inventions is the extrudabilitywhich makes it adaptable to molding processes such as liquid injectionmolding at low pressures. The prepared compositions have a viscositysuch that at least 45 grams per minute can be extruded through a 3.175millimeter orifice under a pressure of 620 kilopascals. Preferably, theviscosity is such that at least 50 grams per minute can be extruded.

The silicone elastomeric compositions can readily be prepared inconventional mixing equipment because of its fluid nature. The order ofmixing is not critical if the composition is to be used immediately.However, it is preferable to combine (A), (C) following the acidtreatment and thereafter add (D) and (B). This permits the small amountof (D) to become well dispersed in (A) and (C) prior to the beginning ofany curing reaction. Suitable two package composition can be made usingsuch as technique. For example, a convenient two package composition canbe prepared by mixing part of acid treated mixture of (A) and (C) andall of (D) in one package and the remainder of acid treated (A) and (C)and all of (B) in a second package such that equal amounts of packageone and package two can be mixed to produce the compositions of thisinvention. Single package compositions can be prepared by mixing(A),(B), (C), (D), (E) and a platinum catalyst inhibitor. Theseinhibited compositions can be stored for extended periods of timewithout curing, but the compositions will still cure when heated above70° C., preferably when heated above 100° C. to shorten the cure time.

In order to demonstrate various features of this invention, thefollowing examples are submitted. They are for illustrative purposes andare not intended to limit in any way the scope of this invention.

EXAMPLE 1 Test Specimen Preparation

A silicone LIM base compound was prepared according to the teachings ofthis invention using the formulation of Table I.

                  TABLE I    ______________________________________    64.5 pts    40,000 cps vinyl chainstopped polydimethylsiloxane                polymer    25   pts    325 m.sup.2 /gm octamethylcyclotetrasiloxane treated                fumed silica or 300 m.sup.2 /gm dimethyldichlorosilane                treated fumed silica    1    pt     vinyltriethoxysilane    6    pts    hexamethyldisilazane    3    pts    water    3    pts    acid solution    4    pts    500 cps vinyl chainstopped, polydimethyl,                methylvinyl copolymer    4    pts    500 cps trimethylsilyl and dimethylvinyl chainstopped                polydimethylsiloxane polymer    2.5  pts    MQ resin    ______________________________________

The 40,000 cps vinyl chainstopped polymer, 3 parts. water andhexamethyldisilazane were mixed together in a cooled mixer. The 325 m²/gm D₄ or 300 m² /gm dimethyldichlorosilane treated filler was addedslowly and mixed until it was completely incorporated. After all thefiller was incorporated, the vinyltriethoxysilane was added and mixedwell. The mixer was sealed and heated for 1 hour at 70°-80° C. The acidsolution was added and the mixture was cooked for 1 hours at 60°-88° C.The batch was stripped at 140° C. under full vacuum to remove all thefiller treating reaction by products and then cooled to 80° C. The two500 cps vinyl containing copolymers were added and mixed well. 2.5 ptsof the MQ resin release agent was added. Pulled vacuum to deair thebatch.

Component A was prepared by adding sufficient amount of Karstedtplatinum organosiloxane complex to obtain 20-40 ppm Pt as platinum.Component B was prepared by adding approximately 330 ppm H of hydridecrosslinker (M^(H) D_(x) D_(y) ^(H) M^(H)) and approximately 0.4 partsmethyl butynol, mixed until well dispersed. A LIM composition wasprepared by mixing 100 parts of component A with 100 parts of componentB in a static mixer with no air being introduced. The A/B mixture wasthen molded 20 seconds at 375° F. into 3"×5"×0.070" sheets.

EXAMPLE 2

A sheet prepared according to Example 1 was post baked for one hour@400° in an air circulating oven and cooled to room temperature. Thesample was referred to as PBO. A second sheet was wrapped in aluminumfoil and post baked under the same conditions. The sample was referredto as PBS. The results are shown below:

    ______________________________________                  PBO       PBS                  1 hr at 400° F.                            1 hr at 400° F.                  ppb DMNA* ppb DMNA*    ______________________________________    3 pts H.sub.2 O (control)                    4.0         42.8    3 pts H.sub.2 O (Extended Cook)                    2.7         9.7    3 pts 1N HCL    <1          <1    3 pts 0.5N HCL  <1          2.2    3 pts 0.25N HCL <1          2.3    3 pts 0.10N HCL <1          2.2    3 pts 1N HC.sub.2 H.sub.3 O.sub.2                    <1          7.5    ______________________________________     *DMNA is dimethylnitrosamine

The results clearly indicate that the standard 3 parts H₂ O when cookedat an extended cook cycle lowered the DMNA level but does not totallyeliminate them. 1N HCL totally eliminated the DMNA, whereas 0.5N, 0.25N,and 0.1N HCL eliminated the DMNA when post baked open, but the sealedresults indicates that the part is cured substantially free of DMNA. 1NHC₂ H₃ O₂ also eliminated DMNA when post baked open, but somenitrosamines were generated when post baked sealed.

EXAMPLE 3

Same materials as described in the above examples except 300m² /gmdimethyldichlorosilane treated fumed silica was used and the acidsolutions were added as noted. This filler is not HMDZ in-situ treatedas opposed to the in-situ filler treatment in Example 2. Samples werepost baked 1 hr at 400° F. in air.

    ______________________________________                   PBO       PBS                   1 hr at 400° F.                             1 hr at 400° F.                   ppb DMNA  ppb DMNA    ______________________________________    3 pts H.sub.2 O no additional filler                     <1          2.0    treatment    3 pts H.sub.2 O filler and HMDZ                     1.2         13.1    treatment    filler + HMDZ + 0.1N HCL                     1.7         11.0    filler + HMDZ + 1N HCL                     <1          <1    ______________________________________

The results clearly indicate that 3 parts H₂ O without additional fillertreatment substantially eliminated the DMNA. 3 parts H₂ O with fillerand HMDZ treatment reduced the DMNA to 1.2 ppb when post baked open, butthe sealed result yielded at 13.1 ppb when post baked sealed. 1N HCLtotally eliminated the DMNA, whereas 0.1N HCL reduced the DMNA when postbaked open. The sealed results yields 11.0 ppb which some DMNA is stillbeing generated.

What is claimed is
 1. A method for making nitrosamine-free siliconearticles comprising the following steps:(a) mixing (A) avinyl-containing organopolysiloxane with (C) a filler; (b) adding anamount of acid solution to the mixture of said step (a) effective toeliminate nitrosamine, (c) adding (B) a silicon hydride siloxane orsilicon hydride siloxane fluid and (D) a catalytic amount ofhydrosilation catalyst to the mixture of said step (b) and cooking saidmixture at temperatures of from about 50° to 100° C. for 0.5 to 2.0hours; (d) curing the mixture of said step (c); and (e) post baking thecured mixture of said step (d).
 2. The method of claim 1, wherein theacid comprises HCl, HBr, HI sulfuric, formic acid, acetic acid orphosphoric acid.
 3. The method of claim 1, wherein the mixture is cookedat temperatures of from about 55° to about 90° C. for 0.5 to 1.5 hours.4. The method of claim 3, wherein the mixture is cooked at temperaturesof from about 60° to about 85° C. for one hour.
 5. The method of claim1, wherein the mixture is cured in a heated molded.
 6. The method ofclaim 1, wherein said step (c) further comprise the step of adding aneffective amount of a catalyst inhibitor.